We examined whether mesangial cells from normal mice synthesized IGF-I binding proteins in vitro. The supernatant from mesangial cells contained a single species of binding protein with a molecular weight of 30 KD. This protein was not glycosylated. It was found to be released in much higher amounts in sparse cells than in confluent cells. It was also found that the cells expressed MRNA for this binding protein, known as BP2. The binding protein release was coordinately regulated with the production of IGF-I by the cells. This new factor could participate in' the control of mesangial proliferation and diseases associated with an increase in glomerular cell turnover could be partly due to a dysregulation in IGF-I-BP production and release.